Expansion joint for gas-insulated transmission line

ABSTRACT

An expansion joint for the central conductor of a compressedgas-insulated transmission line permits the desired mutual longitudinal movement of adjacent central conductors by means of flexible connectors so arranged as to provide a negligible increase of diameter of the central conductive path while having sufficient length and flexing properties such that fatigue is reduced. The flexible connectors are a multiplicity of thin strips disposed in a circular path within a thin surface transverse to the axis of the transmission line.

Unite States Patent [191 Christofferson EXPANSION JOINT FOR GAS-INSULATED TRANSMISSION LINE Inventor:

[75] James Christotferson, West Newbury, Mass.

[73] Assignee: High Voltage Power Corporation,

Westboro, Mass.

Filed: July 14, 1972 Appl. No.:'272,083

US. Cl. ..174/l3, 174/86, 174/99 E Int. Cl. ..H02g 15/24 Field of Search..174/12 R, 13, 21 CA,

References Cited UN lTED STATES PATENTS 11/1969 Hus ..174 13ux May 1,1973 3,548,071 12/1970 Bahen, Jr. et a1 174/13 3,573,342 4/1971 Graybillet a1. ..174/13 X 3,585,270 6/1971 Trump 174/13 Primary Examiner-LaramieE. Askin Attorney-Russell & Nields [57] ABSTRACT 4 Claims, 8 DrawingFigures EXPANSION JOINT FOR GAS-INSULATED TRANSMISSION LINE BACKGROUNDOF THE INVENTION It is well known that a significant trend exists towardthe transmission and utilization of ever larger amounts of electricpower. At the present time, the transmission of power to large urbanpopulation centers has conventionally been by means of overhead linesfrom the point of generation to the point of distribution and usage. Thefew exceptions to this reside in the furnishing of power to downtownareas and in some of the newer and more exclusive suburban areas. In theformer instance, underground power transmission has been a necessity asa result of crowded conditions, limited easements and right-of-ways, andsafety factors where higher voltage and power loads have been required.In the latter instance, however, the driving force behind undergroundcables has been more due to aesthetic continuity of the living area asopposed to technical and practical limitations as the public has beenreluctant to accept the influence of high voltage overhead transmissionlines. In addition, there are both technical and economic limitations tothe underground transmission of small amounts of power by conventionalhigh voltage oil-paper and solid dielectric cables.

The solid-insulated cable is typical of the underground cable now inuse. This cable comprises an inner conductor having a solid insulationbuilt up from spiral-wrapped paper and subjected to a vacuum-dryingprocess followed by thorough impregnation with insulating oil or similarvoid-filling material. These solid insulated cables are used forvoltages as high as 345 Kv. but are inefficient since they carry only afraction of the current capability of an overhead line operating at thisvoltage and at far higher cost per unit power transmitted.

For more than a score of years, consideration has been given tocompressed gas-insulated transmission lines as an alternative tooverhead head lines for both preserving the beauty of the countryside,as well as providing increased power transmission at comparable costs.Moreover, the technical aspects of the gas-insulated transmission lineoffers unique advantages for present and future needs of undergroundelectric power transmission. For example, in contrast to oilpaper cablesystems, a rigid gas-insulated transmission line offers:

l. Substantially reduced charging current and increased permissiblelength of line without relative compensation due to the low dielectricconstant of gas which is essentially unity even at high pressures.Moreover, the electrode geometry of rigid concentric conductors can bemade more favorable than in the case of flexible oil-paper cable. Thiscan result in overall reduction in the comparative capacitance by afactor of about four.

2. Low dielectric loss and lower conductor resistance especially whenthe compressed gas is operated below the ionization level. Thus, thedielectric loss under these conditions, even at high gradients, is ofnegligible amount. The use of a rigid high voltage conductor permits andencourages the use of larger cross sections with corresponding gains incurrent and heat transfer capabilities and with greater choice ofconductor materials. As a result, power capabilities surpassing those ofpresent overhead lines of the same voltage are possible.

3. Improved thermal performance arising from the superior heat transferproperties of compressed gas. This is a result of the nondegradingdielectric properties of gas with temperature, the permissible use oftemperature insensitive spacer materials and the lower thermalresistance to earth of the larger external pipe.

4. Voltage insulating properties which can be applied to all presentvoltage levels and which can clearly be extended to 1000 Kv. 10,000megawatt capacities.

However, previous attempts to economically provide such lines have beenfrustrated by structural problems related to manufacture of the line,difficulties in maintaining the proper radial position of the innerconductor within the outer conductor or gas container, and in joining ofadjacent transmission line sections to provide proper rigidity andexclude contamination. In addition, previous gas-insulated transmissionlines have not been able to compensate properly for thermal expensationof the central conductor when under heavy load and problems have arisenin attempts to assure gastight seals between connecting transmissionline sections that will resist leakage even when the insulating gas ispressurized. Then, too, electrical problems have arisen relative toproviding a sufficient electrical connection between the centralconductors, maintaining a predetermined gradient in the insulating spacebetween the conductors, and minimizing sparkover where supports arerequired in the line. It is the failure of the prior art to cope withthese and other factors that has delayed the commercial realization andembodiment of gas-insulated transmission line.

Various proposals for the construction of gas-insulated transmissionlines are set forth in U.S. Pat. No. 3,585,270 to Trump, from which itwill be seen that one of the major problems associated with such linesis the provision of suitable means to compensate for the thermalexpansion and contraction of the central conductor.

Generally, gas-insulated transmission lines will be constructed insections, so that thermal expansion and contraction of the centralconductor can be accommodated by a suitable expansion joint betweenadjacent sections of the central conductor. Several such expansionjoints are shown in said patent to Trump. An expansion joint which ismanufactured at room temperature may, in operation under full load insummer, reach a temperature of F and, if installed in arctic regions,reach a temperature of -40F in winter. A representative expansion jointmight be required to expand by one-half inch from its factory conditionand also to contract by one-half inch from its factory condition, andthe cycle of expansion/contraction might be 300500 cycles per year.Thus, fatigue properties of an expansion joint are significant. In anexpansion joint of the type shown in FIG. 2 of said patent to Trump,comprising a sandwiched array of bands arranged axially with a loop orbulge extending transversely outward,

the flexing is concentrated at the center of the loop.

An important requirement of the expansion joint is that'its effectivediameter be small. The expansion joint must include enough conductingmaterial to carry the current and the added requirement of mechanicalmovement results in some increase in diameter over that of the centralconductor, as shown in FIG. 2 of said patent to Trump. Expansion jointsdesigned to eliminate such increased diameter may encounter otherproblems: the expansion joint shown in FIG. can eliminate such increaseddiameter, but only by increasing flexure concentration at the center ofthe connecting bands. Any increase in diameter at the expansion jointreduces the voltage which can be carried by the transmission line,unless means are provided to increase the diameter of the outer sheatharound the expansion joint, which in turn may introduce undesirablecomplexities.

SUMMARY OF THE INVENTION The expansion joint comprehended by theinvention preserves low diameter increase while avoiding flexconcentration by providing a multiplicity of thinapertured discs eachhavinga radial gap, said discs being arranged parallel to each other andperpendicular to the axis of the transmission line so that opposed edgesat the gap may move axially with respect to one another with negligiblemovement in the transverse dimension. Each apertured disc forms a splitring around which flexing is uniformly spread, and flexing introducesnegligible change in the diameter of the ring.

While particularly useful in gas-insulated transmission lines, theinvention is not limited thereto, but includes expansion joints forelectrical equipment having similar expansion problems, such as isolatedphase bus bars.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view of agas-insulated transmission line having an expansion joint constructedaccording to the invention, the outer sheath being broken away to showthe central conductor;

FIG. 2 is an enlarged view, partly in longitudinal central section alongthe line 22, of the expansion joint of FIG. 1;

FIG. 3 is a section along the line 33 of FIG. 1;

FIG. 4 is a perspective view, somewhat simplified, of the expansionjoint of FIG. ll, showing relative movement of the components of theexpansion joint;

FIG. 5 is a view, similar to that of FIG. 3, showing an alternativeembodiment of the invention;

FIG. 6 is a view, similar to that of FIG. 3, showing another alternativeembodiment of the invention; and

FIGS. 7 and 8 are views, similar to that of FIG. 3, showing stillanother alternative embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to the drawings, andfirst to FIG. 1 thereof, therein is shown a compressed-gas insulatedtransmission line of the general type shown in US. Pat. No. 3,585,270 toTrump. A central conductor is supported within an outer sheath 1 bysuitable insulating supports (not shown), and the space within thesheath 1 is filled with an insulating gas under pressure. The portion ofthe central conductor shown in FIG. 1 comprises two central conductorsections 2, 3 joined by an expansion joint. As shown in FIG. 1, theentire expansion joint of the present invention is enclosed within anouter shield or sleeve 4 whose outer diameter is only very slightlygreater than the outer diameter of the central conductor 2, 3 so that itis not necessary to provide any alternation in the external sheath 1 inthe vicinity of the expansion joint.

The expansion joint of the invention is shown in greater detail in FIGS.24. Referring now to FIGS. 24, each central conductor 2, 3 generallywill comprise a tubular member, since under normal conditions oftransmission of electrical power the skin effect limits the usefulconductor to a region near the outer surface thereof. In arepresentative transmission line useful for voltages of around 235kilovolts and extending up to 345 kilovolts the central conductor mighthave an outer diameter of four inches and a wall thickness of one-halfinch. In accordance with the invention, the end of each centralconductor 2, 3 of the pair to be joined has affixed thereto a matingsection 5, 6 which in the embodiment of the invention shown in FIGS. 24comprises a generally solid semi-cylindrical member, the mating section5 of one member 2 of the pair of conductors 2, 3 cooperating with themating section 6 of the other central conductor 3 of the pair 2, 3 so asto form a generally cylindrical region within which the semi-cylindricalmating sections 5, 6 are capable of Iongitudinal movement with respectto one another (see FIG. 4). Each mating section 5, 6 may have an outerdiameter of 3 1% inches. The mutually adjacent ends of the centralconductor and mating sections are suitably chamfered and then weldedtogether .(FIG. 2). Each of the mating sections 5, 6 may be providedwith a semicylindrical depression 7, 8 in the flat face thereof in orderthat the pair of mating sections may be mounted on a mandrel foralignment thereof during affixation of the split rings as hereinafterdescribed.

Theexpansion joint itself may conveniently be assembled prior to weldingto the central conductors. In accordance with the invention the matingsections 5, 6 are connected to one another by means of a series of splitrings 9 each lying in a plane transverse to the axis of the transmissionline. Each ring 9 is relatively thin in the axial dimension so that theends of any ring may move axially with respect to one another with aresultant flexing of the ring along its entire length and notconcentrated at any particular region thereof. Depending upon the axialthickness of each ring, a suitable number of rings are provided in orderto enable the family of rings to carry the necessary current passingfrom one mating section to the other. For example, if the axialthickness of each ring is one-sixteenth of an inch, 275 rings may beemployed. Since the rings are to wrap around the mating sections in theembodiment shown in FIGS. 2-4, the inner diameter of each ring 9 must begreater than the outer diameter of the mating sections 5, 6 and may forexample be 3 inches. The outer diameter of the rings 9 is desirably keptas small as possible in order to maintain proper insulation in thetransmission line without the need for changes in the diameter of theouter sheath 1 around the expansion joint. A suitable outer diameter forthe rings shown in the embodiment of FIGS. 24 might be 4 V4 inches.Since it is generally desirable to avoid sliding contact between thecomponents of the transmission line (because of resultant creation ofparticles of aluminum due to rubbing action) one preferably provides aspace such as 0.010 of an inch between adjacent rings 9.

The split rings 9 may be affixed to the mating sections 5, 6 in any of avariety of ways and for this purpose it may be desirable to provide asuitable tab 10, l l on the mating sections 5, 6. In theembodiment shownin FIGS. 2-4 each mating section 5, 6 may, for example, be extrudedaluminum having a cross section shown in FIG. 3 wherein a tab 10, 11 isprovided having a taper for the purpose of providing space for aweldment between one end of the split ring and the mating section. Otherpossible connections such as dip-brazing will be described hereinafter.

Once the split rings 9 have been welded or otherwise affixed to themating sections 5, 6, the expansion joint is ready for connectionbetween the central conductors 2, 3. As shown in FIG. 4 the rings areaffixed to each mating section extending from one end thereof down tosome distance from the other end thereof, and the exposed region of onemating section is at the opposite end from the exposed region of theother mating section. In the representative example under discussion therings themselves together with the spaces between them might occupy alength of inches with a 2 inch length of exposed mating section oneither side of this.

In assembling the expansion joint to the central conductors a ringelement 12 having a rounded side and a notched side is slipped over theend of one 2 of the conductors and the notched side is welded to thecentral conductor 2 at, for example, one-half inch from the end of thecentral conductor 2, the ring element 12 itself having an axial lengthof about 1 inch. The expansion joint may then be welded to that end ofthat central conductor 2. A shield 4 which may comprise a 26-inch longsleeve of aluminum having a thickness of one-sixteenth inch is thenslipped over the expansion joint and fitted into the notch of the ringelement 12 and then welded thereto on the outside, thereby covering andelectrically shielding the weld previously made between the ring element12 and the central conductor 2 as well as the weld between the centralconductor 2 and the expansion joinLThe function of this sleeve iselectrostatic shielding and therefore it may be thin. It mayconveniently have a diameter of 4 k inches which is a very smallincrease over the 4 inch outer diameter of the central conductor 2itself. This sleeve may fit fairly snugly over the rings 9, and with thedimensions given the outer diameter of 4 A inches of the rings will fitwith room to spare within the inner diameter of 4 inches of the sleeve4.

The other central conductor 3 may be welded to the expansion jointbefore the sleeve 4 has been slipped over the joint. There is welded tothe sleeve 4 a second ring element 13 identical to the first except thatin the inner periphery thereof a groove is provided adapted to receive aTeflon O-ring 14. This second ring element 13 slides over the secondcentral conductor 3 and the Teflon O-ring 14 provides a bearing surfacepermitting sliding movement between the second conductor 3 and thegrooved ring element 13.

In the embodiment shown in FIG. 5 the mating sections 5, 6 are providedwith tabs 15, 16 which have a slot adapted to receive the ends of thesplit rings 9. After mechanical assembly of all the rings 9 on themating sections 5, 6 the entire assembly may be dip-brazed in oneoperation.

In the embodiment of FIG. 6 the mating sections l7, 18 have enlargedsemi-cylindrical depressions so as to provide a cylindrical spacebetween the mating sections l7, 18 adapted to receive split rings 19 ofrelatively small outside diameter. The ends of the rings 19 are affixedto tabs 20, 21 extending inwardly from the respective mating sections17, 18.

In the embodiment of FIGS. 7 and 8, FIG. 8 is a cross-sectional viewparallel to that of FIG. 7, but shows split ring 23 which is adjacent toring 22 of FIG. 7. Ring 22 is attached at one end to mating section 24at the upper tab 26, then substantially encircles the perimeter ofmating sections 24 and 25 and is attached at the other end to the uppertab 27 of mating section 25. The connections 28 and 29, respectively,are similar to the above-described connections 26 and 27.

The manner in which successive connective members of split rings areattached to the mating sections alternates between that of FIG. 7 andthat of FIG. 8. Thus, respecting the longitudinal axis of the centralconductors, the direction of current flow is clockwise in one ring andcounter-clockwise in the next. In this manner, any magnetic fieldsinduced by successive rings tend to cancel each other, and adverseinductive effects are minimized.

Having thus described the principles of the invention together withillustrative embodiments thereof, it is to be understood that althoughspecific terms are employed they are used in a generic and descriptiveterms and not for purposes of limitation, the scope of the inventionbeing set forth in the following claims.

I claim:

1. An expansion joint for high-voltage electric cables, comprising:

a first cable conductor terminating in a first mating section,

a second cable conductor terminating in a second mating section,

said mating sections juxtaposed in overlapping relationship, pluralityof thin flexible, conductive connecting members, each said memberattached to said first mating section and to each said second matingsection, the portion of each said member between said attachments lyingin a thin flat surface perpendicular to the axis of said matingsections, whereby said connecting members flex under mutual axialmovement of said cable conductors while resisting any non-axial mutualmovement thereof.

2. The expansion joint of claim 1 wherein each said connecting member isof the shape of a split ring, one end of said split ring attached tosaid first mating section, said split ring extending therefrom andsubstantially encircling said juxtaposed mating sections, and the otherend of said split ring attached to said second mating section.

3. The expansion joint of claim 2 wherein successive connecting memberssubstantially encircle said juxtaposed mating sections from said firstmating section to said second mating section in opposite directions.

4. An expansion joint for a gas-insulated transmission line comprising,in combination with a first conductor and a second conductor axiallyspaced and forming an elongated cylinder at high potential having a gap,a first mating section affixed to said first conductor, a second matingsection affixed to said second conductor, said mating sections. mutuallyoverlapping, and a. multiplicity of conductive members joining saidmating sections, each such member being thin in the axial direction andoccupying a region close 'to the periphery of the highpotential cylinderdefined by said first and second con-. ductors. v

1. An expansion joint for high-voltage electric cables, comprising: afirst cable conductor terminating in a first mating section, a secondcable conductor terminating in a second mating section, said matingsections juxtaposed in overlapping relationship, a plurality of thinflexible, conductive connecting members, each said member attached tosaid first mating section and to each said second mating section, theportion of each said member between said Attachments lying in a thinflat surface perpendicular to the axis of said mating sections, wherebysaid connecting members flex under mutual axial movement of said cableconductors while resisting any non-axial mutual movement thereof.
 2. Theexpansion joint of claim 1 wherein each said connecting member is of theshape of a split ring, one end of said split ring attached to said firstmating section, said split ring extending therefrom and substantiallyencircling said juxtaposed mating sections, and the other end of saidsplit ring attached to said second mating section.
 3. The expansionjoint of claim 2 wherein successive connecting members substantiallyencircle said juxtaposed mating sections from said first mating sectionto said second mating section in opposite directions.
 4. An expansionjoint for a gas-insulated transmission line comprising, in combinationwith a first conductor and a second conductor axially spaced and formingan elongated cylinder at high potential having a gap, a first matingsection affixed to said first conductor, a second mating section affixedto said second conductor, said mating sections mutually overlapping, anda multiplicity of conductive members joining said mating sections, eachsuch member being thin in the axial direction and occupying a regionclose to the periphery of the high-potential cylinder defined by saidfirst and second conductors.